The invention relates to a glass stem making apparatus, and more particularly to a universal stem mold having one or more replaceable marker inserts disposed within a mold block for imprinting information into the glass of the stem.
The glass stem comprises a number of lead-in conductors (or leads) usually ranging from five to twelve for use with a cathode-ray tube, CRT. Each of the leads extends through a glass disk in a direction perpendicular to the plane of the disk forming a circular array of leads. Typically, the leads comprise three component parts: an outer lead, an inner lead and a central press lead. The leads are round, copper sheathed, 42 percent nickel-iron wire, coated with fused sodium tetraborate. This lead material is commonly known as dumet. Alternatively, a one-piece lead of 52 alloy (52% nickel, balance iron) may be used. A glass fillet surrounds the junction of the outer, press, and inner leads to provide structural strength. Typically, there is at least one and preferably two or more dummy fillets, i.e., fillets without leads therethrough. These dummy fillets serve to provide electrical isolation between a high voltage lead and the next adjacent stem leads.
U.S. Pat. No. 3,201,216 issued to Handmann on Aug. 17, 1965 describes an apparatus or stem machine for producing an electronic tube stem. The apparatus comprises upper and lower portions of a stem mold assembly. Such a stem is usually made by inserting the leads in holes in a rotatable lower portion of the stem mold assembly which is mounted on an indexable turret. A glass cylinder is placed on the lower stem mold assembly around or within the leads (or both) and the leads and then the glass cylinder are heated to melt the glass. The glass cylinder is pressed into a disk in which of the leads are embedded. Each of these operations takes place at an indexed position of the turret. During the heating operation, the leads, the glass cylinder, and the lower portion of the stem mold assembly are rotated to uniformly heat the glass. During the pressing step, the upper portion of the stem mold assembly is pressed down onto the glass cylinder. Usually, the upper mold assembly is either rotated in synchronism with, and in properly indexed relation to, the lower mold assembly, or the lower mold assembly is stopped in a properly indexed position relative to a stationary upper mold assembly. When the upper mold assembly is lowered to the pressing position, the leads extend into lead receiving holes in the upper mold assembly. As is known in the art, a tubulation is usually centrally disposed within the disk to permit evacuation and seal-off of the CRT. While most rotary stem machines have 24 or 30 lower stem mold assembly positions, there usually are only tow, three or four upper mold assembly positions on a stem machine, the actual number reflects the preference of the machine designer. Where there are a plurality of pressing positions., reheating of the stem occurs between each pressing position (or station). The upper mold assemblies differ slightly from station-to-station and can be thought of, for convenience, as progressing from a rough press to a finished press.
In the manufacturing of glass stems for electron tubes such as, CRT's, it is desirable to be able to identify the stem machine on which the stems are made and also the date of manufacture to facilitate the evaluation of stem quality. It is known in the prior art to provide this information by reverse engraving the bottom of one or more screws which are threaded into the side of the stem mold. However, the threads of the screws and the threads formed in the mold deteriorate in a short time and necessitate the replacement of the mold. Accordingly, it is desirable to provide stem identification means on at least the last pressing station of each stem machine without shortening the life of the stem mold.